1) Disassemble PC
2) Hard drive: drill a couple of holes through it, open case to let dirt in, hand it in to recycling centre
3) Mother board and RAM (if latter doesn't fit new PC which is likely): recycling centre
3) DVD, CD, Power Supply: if they still work, keep 'em. You wouldn't want any empty space in your garage, would you !
4) Fans: keep. You can put them into the new PC.
5) Case: the basis for a steampunk extravaganza. I am FURIOUS that I didn't keep my last computer case. I have been looking for a complete case to steampunk for two years now, no luck.....

12 bore or a 410 through the hard drive. pick the bones outta that then!